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Contact: Jane Platt
FOR IMMEDIATE RELEASENovember 5, 1997
GALILEO FINDS ARIZONA-SIZED VOLCANIC DEPOSIT ON IO
Observations taken by NASA's Galileo
spacecraft five months
apart reveal a new dark spot the size of Arizona on Jupiter's
moon Io, indicating that dramatic volcanic activity occurred
during that time.
"This is the largest surface change on Io
Galileo during its entire two-year tour of the Jovian system,"
said Galileo imaging team member Dr. Alfred McEwen, a research
scientist at the University of Arizona in Tucson.
The visible change took place during the five
Galileo's seventh and tenth orbits of Jupiter. The change is
manifested as a dark spot about 400 kilometers (249 miles) in
diameter, surrounding a volcanic center named Pillan Patera,
which is named after a South American god of thunder, fire and
volcanoes. Dark features at the center of the deposits may be new
These changes appear in images taken by the
Galileo, with marked differences between the pictures taken on
April 4, 1997 and September 19, 1997. In June 1997 an active
plume was observed over Pillan by Galileo and the Hubble Space
Telescope with a height of 121 kilometers (75 miles), and both
Galileo and ground-based astronomers observed an intense hot
"Most of the volcanic plume deposits on Io
show up as white,
yellow or red due to sulfur compounds. However, this new deposit
is gray, which tells us it has a different composition, possibly
richer in silicates than the other regions," McEwen explained.
"While scientists knew that silicate volcanism existed on Io from
high temperatures, this may provide clues as to the composition
of the silicates, which in turn tells us about Io's evolution."
"Io is probably primarily composed of
silicates, which is
the type of volcanic rock found on Earth," McEwen added, "but the
extreme volcanism of Io may have led to the creation of silicate
compositions that are unusual on Earth."
The Io images showing the changes in Pillan
reveal alterations in the plume deposit of Pele, the large red
oval southwest of Pillan, which may indicate that both plumes
were active at the same time and interacted with one another. A
dark region southwest of Pele, which appears similar to the
Pillan deposits, has been present since the Voyager flybys in
Io is the most volcanically active body in
the solar system.
Scientists hope to learn more about the fiery satellite when
Galileo continues its studies over the next two years, during a
mission extension known as the Galileo Europa Mission. The
extended mission will include eight additional encounters of
Europa, four of Callisto, and two close Io flybys in late 1999,
depending on spacecraft health. Galileo will pass very close to
Pillan Patera in the first of the two Io flybys, so high-
resolution images can be acquired over a small portion of this
Galileo was launched in 1989 and entered
Jupiter on Dec. 7, 1995. The final satellite encounter of its
two-year primary mission will occur on Thursday, Nov. 6, 1997 at
12:32 p.m. PST, when the spacecraft swoops over Europa at an
altitude of 2,042 kilometers (1,269 miles).
"The Galileo orbiter is performing flawlessly
and all 11 of
its sophisticated science instruments and the radio science
investigations are still providing excellent data," said Galileo
Project Manager Bill O'Neil of NASA's Jet Propulsion Laboratory,
Pasadena, CA. "A great bounty of Jupiter system science has been
obtained and the continuing study of these data will surely add
many important discoveries. While not all of the original
objectives could be met due to the antenna failure, I believe
that the overall science return from Galileo will easily exceed
what was envisioned at project inception 20 years ago, because
our team of scientists and engineers has done such a superb job
of capturing the most important observations."
The Galileo mission is managed by JPL for
NASA's Office of
Space Science, Washington, DC. JPL is a division of California
Institute of Technology, Pasadena, CA.
Images of Io and other data received from the
posted on the Galileo home page at